Low-cost conveniently synthesized fuels with diesel-like characteristics have been long sought. However, difficulties such as low lubricity, high volatility and low energy content have limited their adoption. The source for such alkanes can be natural gas, coal gas, biogas or from partial oxidation of chemical waste such as plastics.
Oxygenate blends rich in alcohols and aldehydes are produced by well-known methods such as the partial oxidation of alkanes. It is desirable to form a product capable of being utilized directly in a compression ignition engine from these mixtures, rather than undertake the more circuitous methanol olefins to gasoline and distillates (MOGD) approach for conversion of oxygenates to diesel. Moreover, the organic content of such a fuel product can be produced entirely from the blend with sufficient efficiency to be sold at a cost less than that of traditional diesel on an energy equivalent basis. Large diesel demands and scarce fuel supplies are typical of stranded areas where associated gas is flared so in the case of partial oxidation GTL processes this invention meets a critical need to minimize fuel transportation.
Although lower alcohols such as methanol and ethanol are typical of direct partial oxidation of alkanes, neat methanol is not a suitable fuel in compression ignition internal combustion engines as it has a low cetane number, meaning that it is difficult to self-ignite under compression and is therefore unsuitable for usage in a diesel engine. One solution is to react the aldehydes with the alcohols in situ to form acetals such as dimethoxy methane. Dimethoxymethane, synthesized from formaldehyde and methanol, is generally considered too volatile for use in diesel engines although it has suitable ignition characteristics for compression ignition engines. It also possesses unfavorable lubricity characteristics. A solution to this problem is to selectively form acetals by reacting alcohols with carbons greater than that of ethanol with formaldehyde and higher aldehydes. However, there is another significant problem encountered with such acetals. Although the alkoxy groups of formaldehyde dialkyl acetals, corresponding to the Cn alkanols from which they were derived, where n is the carbon number from 2 to 4, have been shown to be very diesel like in terms of viscosity, cetane number, flash point, and lubricity, they are known to form peroxides. (Murphy, M., Safety and Industrial Issues Related to the Use of Oxygenates in Diesel Fuel. SAE technical paper 1999-01-1473, 1999). Diethoxy ethyl acetal, commonly known as acetal, is particularly prone to this problem.
Hydroperoxides tend to form at the CH bond adjacent to the oxygen of the alkoxy functionality of ethers and acetals. These ethers are particularly dangerous on the higher substituted carbons adjacent to the ether (Brown et al. Organic Chemistry 5 ed.). For example, diisopropyl ether, having tertiary substitution at this carbon site, has a history of dangerous explosions whereas dimethyl ether with only primary substitution at this carbon is generally not known for forming peroxides. Likewise, as previously mentioned, (1,1 diethoxy ethane) is known to possess significant risk for peroxide formation. So far this problem has not been addressed in regards for a purely oxygenated diesel fuel substitute.
U.S. Pat. No. 2,130,080 (the '080 patent) discloses a composition which inhibits the formation of peroxides on dialkyl ethers following exposure to atmospheric oxygen. The disclosed additive is a carbonyl group such as a ketone or carboxylic acid with aliphatic substituents. The specific peroxide inhibitors mentioned are acetone, methyl ethyl ketone, or salicylic acid. The '080 patent does not disclose the application of these inhibitors to inhibit peroxide formation on a fuel blend. Moreover, the '080 patent fails to teach the use of such compounds to inhibit peroxides on acetals.
U.S. Pat. Pub. No. 2007/0130822 discloses a biofuel composition containing dimethoxymethane, however, no claim is made of the use of higher acetals. Furthermore, no mention is made with regards to the tendency of some of the mentioned oxygenates to form dangerous peroxides, nor is the addition of additives with substantial antioxidant properties disclosed. In addition, many of the oxygenate additives are not readily synthesized from the products that result from direct partial oxidation, nor is direct partial oxidation mentioned as a source of such oxygenates.
U.S. Pat. No. 7,615,085 (the '085 patent) discloses a mixture of acetals and/or carbonates and/or esters to be blended with low sulfur diesel and an antioxidant additive to inhibit peroxide formation. The oxygenate disclosed therein is a trialkyl substituted carbon in the beta position to a polar group such as OH, aldehyde, ketone, nitro functionality. However, the '085 patent does not disclose the use of dialkyl acetals higher (in the sense of the number of carbons in the aldehyde or ketone) than dimethoxymethane. Furthermore, the '085 patent discloses blending of oxygenates at a concentration of 500 to 2500 parts per million of fuel (v/v). Alcohols and simple dialkyl ethers are not specified as being part of the oxygenated blend. Moreover, the '085 patent fails to teach that ignition enhancing ability of an additive containing nitro (R—CH2—NO2, R being an alkyl group of tertiary substitution of 2 to 20 carbons) or ketone functionality for lower alcohols. Finally, the '085 patent does not teach an efficient synthesis of oxygenates or the additive synthesized from the materials inherent with direct partial oxidation.
Lower alcohols have traditionally been rendered suitable for usage in compression ignition engines when supplemented with ignition enhancers that raise the cetane number of such mixtures. Prior art ignition enhancers for methanol include triethyleneglycol dinitrate, octyl nitrate, cyclohexyl nitrate, 2-n-butyoxyethyl nitrate, 2-methoxyethyl nitrate, and tetrahydrofuryl nitrate. The synthesis of such compounds does not involve materials readily obtained through direct partial oxidation. Some prior art ignition enhancers for specifically improving ethanol blends include long chain polyethylene glycols. These compounds are also not readily synthesized from direct partial oxidation products. Moreover, the cost of these specialized compounds has been prohibitive for wide scale adoption of methanol as a diesel fuel in compression ignition engines. Finally, experience has shown that nitrogen emissions are not substantially increased by the usage of such nitrated compounds.
Accordingly, there is a need for improved diesel fuels that can be economically synthesized and which lack the said drawbacks.